Solar cells are standing out as an eco-friendly clean energy and are in practical use. A solar cell containing a crystalline silicon is now being extensively used. A problem with this solar cell is high costs of power generation due to use of a highly pure silicon. Another problem is inefficiency in conversion of weak light (such as indoor weak light).
In order to solve these problems, a solar cell containing an organic material in a photoelectric conversion site is being widely developed. In particular, a dye-sensitized solar cell is attracting much attention. The dye-sensitized solar cell was developed by Graetzel et al. in Swiss Federal Institute of Technology in Lausanne [for example, Japanese Patent No. 2664194 (JP-2664194B, Patent Document 1)]. It is a great characteristic of the dye-sensitized solar cell to contain a metal-oxide-semiconductor (e.g., titanium oxide) and a sensitizing dye as a photoelectric conversion site.
Conventional solar cells share a problem in terms of stable supply of electric power. For example, each one of the solar cells generates electricity only under the exposure of the light and does not function as a cell during the night. In addition, the generating power (or output power) fluctuates depending on the intensity of the light. In order to solve these problems, the solar cell is sometimes used in combination with an electricity storage apparatus (a capacitor or a condenser).
The production of the dye-sensitized solar cell with the electricity storage function is also being attempted. For example, Japanese Patent No. 4757433 (JP-4757433B, Patent Document 2) discloses a rechargeable solar cell. The rechargeable solar cell has a first electrolyte solution, a second electrolyte solution, and a cation exchange membrane interposed between the first and second electrolyte solutions; the first electrolyte solution and the second electrolyte solution are cut off from an external air; the first electrolyte solution contains iodine and an iodine compound; the second electrolyte solution contains an iodine-free compound; the components of the first electrolyte solution are different from the components of the second electrolyte solution; there are a photoanode and a counter electrode in the first electrolyte solution; there is a charge storage electrode in the second electrolyte solution; and the photoanode and the charge storage electrode are separated from each other by the cation exchange membrane.
Unfortunately, the dye-sensitized solar cell described in this document requires two electrolytic solutions different in composition from each other in order to impart an electricity storage function and inevitably has a highly complicated structure due to a newly added electrode. Moreover, use of a polypyrrole or the like as the positive electrode increases the internal resistance and is thus a factor of decrease in the generating power of the dye-sensitized solar cell. In addition, since an electrode to be used for the generating power from the solar cell differs from an electrode to be used for the generating power (discharge) from the electric double layer capacitor, it is necessary to control the generating powers by an external circuit. Thus the solar cell has a complicated circuit structure.
Meanwhile, in a dye-sensitized solar cell, the photoelectric conversion occurs on an interface between a metal-oxide-semiconductor and a sensitizing dye. In order to increase the photoelectric conversion efficiency, it is desired to increase the surface area of the metal-oxide-semiconductor. Thus, for the dye-sensitized solar cell, an electrode composed of a nano-sized metal-oxide-semiconductor is used to increase the effective area compared with the apparent area.
In a case where the metal oxide nanoparticle is just applied on a substrate, the metal oxide nanoparticle easily peels (or separates) from the substrate due to a slight impact strength and thus fails to function as an electrode. Moreover, since the generated electricity cannot be drawn out efficiently due to a large electric resistance between the particles, the resulting cell has a low conversion efficiency. These problems are solved by applying (or coating) a titanium oxide nanoparticle on a substrate and then heat-treating the coated substrate at a high temperature (about 450° C.) to melt-bond the titanium oxide particles.
Unfortunately, this method, which requires the exposure of the substrate to a high temperature, restricts the substantially practicable substrate to an inorganic material (e.g., a glass). Thus this method cannot produce a flexible dye-sensitized solar cell containing a plastic substrate.
Moreover, since the dye is thermally decomposed in the sintering (heat-treating) step, the dye cannot be adsorbed on the metal-oxide-semiconductor before the application (or coating). Thus a dye-adsorbing step is necessary after the sintering step. On the whole, this method requires complicated processes, including the sintering step, which is one of factors increasing the production cost.
Japanese Patent Application Laid-Open Publication No. 2005-251426 (JP-2005-251426A, Patent Document 3) discloses a method for measuring an amount of a dye, the method comprising the steps of: fixing a metal oxide, a metal sulfide, a metal nitride, a metal cluster, or an alloy thereof on a conductive substrate so that a dye can be coupled detachably on the substrate; coupling a dye to the substrate; irradiating a light with the resulting substrate to generate a current; measuring the amount of the current; and determining the amount of the coupled dye from the amount of the current. This document discloses that the method for fixing the metal compound so as to allow the isolation of the dye preferably includes use of a polymer electrolyte and that, in Examples, Nafion® (manufactured by Aldrich, trade name “Nafion 117”, average molecular weight: 1000) was suspended in 1 ml of ethanol, 400 ml of a 20.5% aqueous solution of titanium oxide fine particle (manufactured by TAYCA Corporation, trade name “TKS-203”, particle diameter: about 6 nm) was uniformly dispersed in the suspension, and the resulting titanium oxide-Nafion sol dispersion was used to produce an ITO electrode modified with titanium oxide.